1. Field of the Invention
The present invention relates to a method for manufacturing a honeycomb structure forming die for extrusion-forming a honeycomb structure having cells having different sizes.
2. Description of Related Art
There is increasing the need for removing particulate matter and harmful substances in exhaust gas from internal combustion engines, boilers, and the like in consideration of influences to an environment. In particular, regulations regarding the removal of Particulate matter (hereinbelow sometimes referred to as “PM”) discharged from diesel engines tend to be strengthened on a global basis, and the use of a honeycomb filter is attracting attention as a trapping filter (hereinbelow sometimes referred to as “DPF”) for removing PM. Therefore, various systems have been proposed. In the aforementioned DPF generally has a structure where a plurality of cells having a cross sectional shape of a quadrangle, hexagon, or the like and functioning as fluid passages are formed by porous partition walls and where the porous partition walls constituting the cells play a role of a filter. Here, the “cross sectional shape” means the shape of a cross section taken along a plane perpendicular to the longitudinal direction of the cells.
In the DPF, a target fluid to be treated, which contains particulate matter, is allowed to flow in the DPF from one side end portion, and, after the particulate matter is filtrated by the partition walls, purified gas is discharged from the other side end portion. There is a problem that, as the exhaust gas flows in the DPF, the particulate matter contained in the exhaust gas deposits in the one side end portion (exhaust gas inflow side end portion) to clog a cell. This is prone to be caused in the case that a large amount of particulate matter is contained in the exhaust gas or in a cold district. When a cell is thus clogged, there arises a problem that the pressure loss in the DPF is rapidly increased. In order to suppress such clogging of a cell, there has been proposed a structure (HAC (High Ash Capacity) structure) where the cross-sectional area of the cells (inflow side cells) open in the end portion on the exhaust gas inflow side is different from that of the cells (outflow side cells) open in the other side end portion (end portion on the exhaust gas outflow side) (see, e.g., JP-A-2004-896). Here, the cross-sectional area means the area of a cross section taken along a plane perpendicular to the longitudinal direction of the cell. By increasing the size (area) of the cell opening portion (cell cross section) of the inflow side cells than that of the outflow side cells, the surface area of the inflow side cell surface where particulate matter and the like deposit becomes large, which enables to suppress the increase of pressure loss.
As a method for manufacturing a ceramic honeycomb structure, there has conventionally been known a method using a honeycomb structure forming die (hereinbelow sometimes referred to as a “die”) provided with back holes (hereinbelow sometimes referred to as an “introduction holes”) for introducing a forming raw material (hereinbelow sometimes referred to as “kneaded clay”) and a die substrate where slits having a hexagonal shape or the like and communicating with the back holes are formed. The forming raw material such as a ceramic raw material introduced in the die from the back holes moves to the slits having narrow width from the back holes having relatively large inner diameter and is extruded as a formed article having a honeycomb structure (honeycomb formed article) from the opening portions of the slits.
As a method for manufacturing such a honeycomb structure forming die, there has been known a grinding method in the case of quadrangular cells. In addition, as a method for manufacturing a hexagonal cell honeycomb structure forming die, for example, there is disclosed a manufacturing method where the aforementioned honeycomb shaped slits are formed by electro-discharge machining (EDM) (see, e.g., Japanese Patent No. 1784822, Japanese Patent No. 1784823, and JP-A-2002-273626). As a method for forming slits in a hexagonal cell die, there is disclosed a method where each side of the cell is worked by a rib electrode of carbon or the like (see JP-A-63-28522). In addition, in order to stably perform slit machining by duplicating discharge, there is disclosed a method where holes for allowing a processing liquid to flow therethrough in positions corresponding with back holes are made before slit machining (JP-A-2005-254345).
However, in a honeycomb structure having different cell sizes between the target fluid inflow side and the outflow side, since cells having different cell sizes are aligned, the partition walls are not linearly formed. Therefore, the slits for forming partition walls of the honeycomb structure in the honeycomb structure forming die for extrusion-forming a forming raw material are disposed not linearly but in a concavo-convex shape. Therefore, it is impossible to form the slits in the honeycomb structure forming die having such a structure by grinding as in the die for extrusion-forming a honeycomb structure of quadrangular cells.
Therefore, in order to form slits corresponding with the cell structure having different cell sizes between the target fluid inflow side and the outflow side, a die has been manufactured by electro-discharge machining using an electrode having a mesh shape which is like a transcription of a cell structure of the slits. However, since many steps are required for manufacturing a mesh-shaped electrode and a DPF has a large diameter, time of several months to half year is required for manufacturing an electrode for manufacturing a die.
Also, a HAC die of a DPF has the following problem. Since the DPF has a large rib thickness, the slit width of the die is wide, the machining removal amount is increased, and the number of steps is increased. Since a sludge generation amount is increased according to the increase in the processing removal amount, in a method where liquid holes are formed and where a processing liquid is allowed to flow through the liquid holes (see JP-A-2005-254345), machining removal is difficult because the flow rate of the processing liquid is not sufficient. This causes deterioration in processing stability and accuracy of slit width/depth of the slits. In addition, since the liquid holes are formed with a laser or a drill, processing stability of the slits deteriorates by the influences of dross and burr. Therefore, there has been demanded a method for more easily manufacturing a die for extrusion-forming a honeycomb structure having different cell sizes between the target fluid inflow side and the outflow side.